LECTURE 1 Varieties of accelerators Particle Sources ,Linear ...

LECTURE 1 Varieties of accelerators Particle Sources ,Linear Accelerators, Circular Accelerators Accelerator Technologies Magnets, Radiofrequency Systems,Vacuum systems Applications of Accelerators Research Other applications 11/21/01

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Particle Source

Linear accelerator

THE GENERIC ACCELERATOR COMPLEX

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Particle Sources Ion sources:

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DC HV guns: 50-500 keV acceleration Electron production mechanism: • thermionic emission (pulse duration controlled by a pulsed grid) • photocathode irradiation by pulsed laser (laser pulse width determines the pulse duration)

(see N. Augert, “Ion Sources”, Ref. 2, Vol. 2, pp. 619-642) Positive ion sources: the positive ions are formed from electron bombardment of a gas and extracted from the resulting plasma. Species ranging from H to U (multiply charged) are available Negative ion sources: Principal interest is in H- , for chargeexchange injection Surface sources: In a plasma, H picks up electrons from an activated surface Volume sources (magnetron, Penning): electron attachment or recombination in H plasma Polarized ion sources: e.g., optically pumped sources: some penalty in intensity, relatively high (>65%) polarization USPAS Lecture 1

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Electron sources

Varieties of accelerators:

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Circular accelerator

RF guns: cathode forms one wall of an accelerating RF cavity Rapid acceleration to above 10 MeV in a few cells->mitigates space charge effects, makes for low emittance Electron production mechanism: • thermionic emission (pulse duration controlled RF structure) • photocathode irradiation by pulsed laser (laser pulse width determines the pulse duration) 3

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Positron sources (see R. Chehab, “Positron Sources”, Ref. 2, Vol. 2, pp. 643-678) “Conventional” positron source: Can get from 10-3 :1 up to ~1:1 positron/electron as electron energy rises from 0.2 to 20 GeV 0.2-20 GeV Electrons

Target

Antiproton sources antiprotons

30-150 GeV protons

Positrons

to storage rings and stochastic cooling systems

Positrons RF Linac

Target Lens

Matching solenoid

Yield of antiprotons/proton is typically 10 –5

Solenoid

Positron production by high energy photons >100 GeV electrons

photons

positrons RF linac

Converter electrons

helical undulator

Sweep magnet

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Linear Accelerators RF Linacs (see M. Weiss, “Introduction to RF linear accelerators”, Ref. 2., Vol 2, pp. 913-954)

Electrostatic Accelerators: Tandem Van de Graff, Pelletron Ion beam, energy 2qV

Magnetic focusing lens

HV terminal is charged mechanically

Stripping foil

Partcle beam ..............

HV terminal at potential +V

Energies are limited to 10-20 MeV RF cavity

Lenses are required for transverse stability (the RF cavities are transversely defocusing) Negative ion source charge -q

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RF cavities may be of two forms: “travelling wave” or “standing wave”

For a uniform waveguide, the phase velocity is greater than c, so the particle gets out of sync with the wave very rapidly. The waveguide must be loaded with periodic obstacles (disks) , with holes (irises) for the beam

Travelling wave cavity: RF in

LOAD

RF out

LOAD RF out

RF in

Waveguide beam particle

Typically operated in a TM mode: longitudinal electric field provides the acceleration. Beam is bunched, rides at crest of wave 11/21/01

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Disk-loaded waveguide 11/21/01

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0 mode: fields are the same in adjacent cavities Drift-tube (Alvarez) linac: typically used at the first stage in a proton linac

Standing wave cavity: RF in beam particles

RF in beam particles

Drift tubes

Longitudinal electric field Longitudinal electric field

π mode: fields alternate in adjacent cavities

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Beam is shielded from the fields when at the wrong phase by using hollow “drift tubes”

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Induction linac: Magnetic core (e.g,. metglas) Beam

Acceleration gap

Radio-frequency quadrupole (“RFQ”): A long electric quadrupole, with a sinusoidally varying voltage on its electrodes. The electrode tips are modulated in the longitudinal direction; this modulation results in a longitudinal field, which accelerates particles. It is a capable of a few MeV of acceleration. Typically used between the ion source and the Alvarez linac in proton RF linacs. 11/21/01

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Pulser

Solenoid

The beam forms the secondary circuit of a high-current pulse transformer Induction linacs have very low rep rates (a few Hz) and intermediate voltages (30-50 MeV) but very high peak currents (>10 kA) in short (100 ns-1 µs) pulses 11/21/01

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Circular Accelerators

To get vertical focusing, field B must decrease with ρ:

Cyclotrons (see P. Heikken, “Cyclotrons”, in Ref. 2, Vol. 2, pp. 805-818) Classical cyclotron: Static B field, DC beam RF: Voltage V, frequency f Dees Magnetic field B

Pole tips Magnetic field

Centripetal force=Lorentz r r force= e[ E + vr × B]

ρ

2

ρ

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Particle orbit

mv = evB ρ mv p ρ= = eB eB v eB f = = 2πρ 2πm

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Force

This fact, and the relativistic mass increase with velocity, limits the maximum energy of the classical cyclotron to about 10% of the rest mass energy.

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Improvements:

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Synchrotrons v

e

• Isochronous cyclotron: Vary the B field azimuthally. This gives vertical focusing (like alternating gradient focusing in a synchrotron) and the B field can then increase with ρ to keep f constant as m increases.

RF Cavity

ρ

bending fields focusing fields

• Synchrocyclotron: Increase the RF frequency on the dees as m increases to maintain synchronism. The beam must then be bunched, as only a limited region in RF phase is stable, and the current is reduced.

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p . As p increases during eB acceleration, B is increased to keep ρ constant. The RF frequency v f= is constant for relativistic particles. 2πρ

This can be represented as n  ρ0  By = B0    ρ where n, called the field index, is > 0. However, n cannot be arbitrarily large, since we must also have radial stability:

A ring of magnets at fixed ρ =

Focusing: to get vertical stability, we could allow the bending field to decrease with ρ, as in the classical cyclotron.

x ρ evBy

Magnetic field

ρ

0

mv 2 ρ

ρ

For radial stability, the centrifugal force must be less than the Lorentz force for x>0:

Force

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mv 2 mv 2 mv 2  x = ≈ 1 −  ρ ρ0 + x ρ0  ρ0  n

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Optical analogy: f1

f2

n

 ρ   x ρ  ≤ evBy = evB0  0  = evB0  0  ≈ evB0 1 − n   ρ ρ0   ρ0 + x  

is positive for a large

d

If this is done by providing a radially varying field in the bending magnets: the machine is called combined function. If this is done by using uniform field dipoles and separate quadrupole magnets, the machine is called separated function. Much greater focusing, hence smaller beam sizes, are obtained in a strong focusing machine than in a weak focusing one.

Focusing of this kind is called “weak focusing”. In 1952, “strong focusing”, or “alternating gradient focusing”, was invented by Courant and Snyder. Strong focusing: alternate the focal length of magnetic lenses around the ring USPAS Lecture 1

1 1 1 d = + − f f1 f2 f1 f2

range of focal lengths and d=> net focusing both radially and vertically

x x ≥ n ⇒1≥ n ρ0 ρ0

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The RF cavities in proton synchrotrons are used primarily to accelerate the particles. However, in electron synchrotrons of moderate to high energies, the RF cavities must also provide the energy lost by the electrons due to synchrotron radiation. (see R. P. Walker, “Synchrotron Radiation”, Ref. 2, Vol. 2, pp 437-460) The energy loss per turn is U0 =

Magnetic field B Particle orbit

ρ

e2 β 3γ 4 3ε 0 ρ

E 4 [GeV] In practical units U0 [MeV] = 0.0885 ρ[ m ] The radiation spectrum extends from the infrared to the many keV region. 11/21/01

Betatron: The circular analog of the induction linac

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Induced electric field

The orbit radius ρ is fixed. The accelerating electric field is provided by the changing magnetic flux within the orbit. The required flux change is ∆φ = 2πρ 2 Bmax . Energies up to 300 MeV have been obtained. 11/21/01

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2π V cos φ , n=1,2,… (for relativistic c particles), φ=relative particle-RF phase

Microtron

Resonance condition: nλB =

(see P. Lidbjork, “Microtrons”, Ref. 2, Vol. 2, pp 971-81) Racetrack microtron:

Accelerator Technology

Magnetic field B

Magnet Systems

Particle orbits

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Principal types of magnets used in accelerators: • Dipoles: provide a “static” transverse uniform field, typically to a few parts in 104 • Quadrupoles: provide a “static” uniform transverse field gradient, to the same accuracy • Sextupoles: provide a “static” quadratic dependence of transverse field on position

RF Linac: voltage V, Wavelength λ

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• Combined function dipoles: provide a mostly uniform transverse field, with a small field gradient • Solenoids: Provide a longitudinal uniform field • The fields are ramped during acceleration in a synchrotron.

“Generation of magnetic fields for accelerators with permanent magnets”, Ref. 2, Vol. 2, pp 893-998) • Resistive electromagnets-the standard solution: excited with copper or aluminum coils, dipole field range up to about 2 T (See N. Marks, “Conventional Magnets”, Ref. 2, Vol. 2, pp 867-912)

Magnet power supplies: Primarily DC or slowly ramping for ring magnets; currents from 10-10,000 A, current regulation required typically 10-100 parts per million. Principal magnet technologies: • Permanent magnets-for fixed-energy rings, dipole fields up to about 1 T. Alnico, ferrite, SmCo3, Nd2Fe14B. Need good temperature regulation for field stability. (See T. Meinander, 11/21/01

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Radiofrequency Systems • Traveling wave and standing wave cavities are both possible elements in rf linacs or synchrotrons. The principal performance parameters of a cavity are its electric field (voltage gradient) and frequency. Generally, the relation between these is Ea λ ≈constant: Higher frequency systems can develop larger voltage gains. Systems from 20 MHz up to 11 GHz are in operation. • Normal conducting (NC) cavities are fabricated from OHFC copper. (See M. Puglisi, “Conventional RF System Design”, Ref. 2, Vol.2, p 677-716)

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• Superconducting magnets-reduced power consumption, higher field range, used in large proton synchrotrons. Dipole fields up to 8 T (with NbTi superconductor), to 15 T (with Nb3Sn superconductor) at 4.2o K. Require extensive cryogenic systems. (See S. Wolff, “Superconducting Accelerator Magnet Design, Ref. 2, Vol. 2, pp 755-790) • Pulsed magnets: provide rapidly varying fields, for beam injection, extraction, and fast switching in transport lines 11/21/01

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• Superconducting (SC) systems typically use pure Nb cavity surfaces (sometimes plated on copper) at 4.2oK. (See H. Lengeler, “Modern Technologies in RF Superconductivity”, Ref. 2, Vol.2, p 791-804) Higher accelerating fields (at low frequencies) are obtainable with SC systems than with NC systems. Power dissipation is much reduced in SC systems, although a cryogenic system is required. Radiofrequency power sources: • Triodes and tetrodes: high power vacuum tube power generators, useful up to 300 MHz and 100 kW

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• Klystrons: Narrow-band, tuned microwave amplifiers--500 MHz to15 GHz 50-100 kV

Electron beam

Catcher

Accelerator vacuum systems are the most challenging for storage rings: synchrotrons in which the beam is maintained at fixed energy for many hours. Required pressures are typically below 10–9 Torr (about 10–12 atmospheres). • Proton storage rings: gas load from thermal outgassing • Electron storage rings: gas load from thermal outgassing and from photodesorption due to synchrotron radiation. (Very high energy proton storage rings also have photodesorption issues)

Cathode Heater Anode Buncher

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RF signal in Drift region

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Vacuum Systems (See A. G. Mathewson, “Vacuum System Design”, Ref. 2, Vol. 2, pp 717-730)

High power RF out

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Applications of Accelerators: Research Pumping systems: Turbomolecular pumps, sputter ion pumps, non-evaporable getter pumps, and titanium sublimation pumps.

High Energy and Nuclear Physics

The pumping system can be lumped for proton storage rings, but must be continuously distributed around the ring for rings with high photodesorption gas loads.

The collider energy advantage:

History: Livingston Plot

E

Fixed target (ultrarelativistic):

Collider: 11/21/01

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E

E

m

m

m

m

Ecm ≈ c 2 Em

Ecm ≈ 2 E

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Luminosity: the other figure of merit (besides energy) of a high energy collider Luminosity (L) = reaction rate per unit cross section Area A

Beam N1 particles

l

Luminosity L =

Target: density ρ, reaction cross section with beam q

Colliding beams: N1

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 r2  dN N exp = − 2 ; then dA 2πσ 2  2σ  N1 N2 4πσ 2

Type

Facility

e+-e-, two rings e+-e-, single ring e+-e-, single ring e+-e-, two rings e+-e-, two rings e+-e-, single ring Pbar-p, single ring e-p, two rings

DAFNE (Italy) BEPC (China) CESR (US) SLAC PEP-II (US) KEK-B (Japan) CERN LEP (Europe) Fermilab Tevatron (US) DESY HERA (Germany) SLAC SLC (US)

e+-e-, linear collider

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Types of high-energy colliders: operating

dL dN dN = f 1 2 dA dA dA for Gaussian, round beams, of rms size σ:

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2

N2 A f is the collision frequency; A is the cross-sectional area of the beams

In differential form:

L= f

N

L = fN1

Number of target particles N2 = ρlA Effective area seen by the beam Aeff = qN2 = qρlA Aeff = qρl Probability of an interaction P = A dR dN1 dN = P = 1 qρl Reaction rate dt dt dt 11/21/01

1 dR dN1 dN N = ρl = 1 2 q dt dt dt A

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CM energy(GeV) 1.05 3.1 10.4 10.4 10.4 200 1,800

Luminosity(1033 cm-2 s-1) 0.01 0.05 0.8 0.6 0.3 0.05 0.02

300

0.02

100

0.002

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Accelerators for nuclear physics (operating) Types of high energy colliders: under construction or proposed

Type

Facility

pp, two rings e+-e-, linear collider µ+-µ-, single ring pp, two rings

CERN LHC NLC, JLC, TESLA, CLIC Muon collider VLHC

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CM Luminosity(10 energy(GeV) 33 cm-2 s-1) 14,000 10 500-1,500 10 100-3,000

0.1-100

100,000

10

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BNL RHIC (US)

CM energy(GeV) 100/nucleon

Luminosity(1033 cm-2 s-1) 10-6

CEBAF (US)

4

---

Bates (US) IUCF (US)

0.3-1.1 0.5

-----

MSU NSCL (US)

0.5

---

TRIUMF(Canada)

0.5

---

PSI (Switzerland)

0.5

---

Type

Facility

AuAu, two ring collider Electron Microtron Electron linac Proton synchrotron Isochronous heavy-ion cyclotron Isochronous cyclotron Isochronous cyclotron

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Applications of Accelerators: Research

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Other applications of accelerators

(See O. Barbarlat, “Applications of particle accelerators”, Ref. 2, Vol. 2, p 841-854)

Field Atomic Physics

Accelerator Low energy ion beams

Condensed matter physics Condensed matter physics Material science

Synchrotron radiation sources Spallation neutron sources Ion beams

Chemistry Synchrotron radiation and sources biology

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Topics of study atomic collision processes, study of excited states, electron-ion collisions, electronic stopping power in solids X-ray studies of crystal structure

Neutron scattering studies of metals and crystals, liquids, and amorphous materials Proton and X-ray activation analysis of materials; X-ray emission studies; accelerator mass spectrometry Chemical bonding studies: dynamics and kinetics; protein and virus crystallography; biological dynamics

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• Oil well logging with neutron sources from small linacs • Archaeological dating with accelerator mass spectrometry • Medical diagnostics using accelerator-produced radioisotopes • Radiation therapy for cancer: X-rays from electron linacs, neutron therapy from proton linacs, proton therapy; pion and heavy-ion therapy • Ion implantation with positive ion beams • Radiation processing with proton or electron beams: polymerization, vulcanization and curing, sterilization of food, insect sterilization, production of microporous membranes • X-ray microlithography using synchrotron radiation • Inertial confinement fusion using heavy-ion beams as the driver • Muon-catalyzed fusion • Tritium production, and radioactive waste incineration, using high energy proton beams

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